gallium: add PIPE_CAP_ESSL_FEATURE_LEVEL

gallium: add PIPE_CAP_ESSL_FEATURE_LEVEL

Adds a new cap to allow drivers to expose higher shading language
versions in GLES contexts, to avoid having to report an artificially
low version for the benefit of GL contexts.

The motivation is to expose EXT_gpu_shader5 even though a driver may
not support all the features needed for the corresponding GL extension
(ARB_gpu_shader5).

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>